1. Field of the Invention
The field of the invention relates to optical recording systems, particularly focus and tracking apparatus usable with such recorders.
2. Discussion of the Prior Art
Every optical recorder that employs elongated record tracks, whether in raster or in concentric-circular form requires the functions of focussing the optics through an objective lens to the medium and for track following and seeking. These combined actions ensure that a light beam from the optics of the recorder is faithfully focussed and aligned with the record track being scanned or to be scanned. Typically, an objective lens is moved or tilted for providing the track-following functions as well as for so-called short seeks; that is, movement of the beam from a track currently being scanned to a track within fifty tracks of the current track. For achieving both focus and tracking functions within a single objective lens support requires that the lens support provide two degrees freedom. A first degree of freedom is along the axis of the objective lens for focussing the lens to the record medium. The second degree of freedom is in a direction orthogonal to the longitudinal extent of the track being followed. Such compound motions require actuators for moving the objective lens along the two degrees of freedom. Both magnetic coils and piezo-electric actuators have been employed for these mounts. An example of magnetic actuators for such two axis control is shown in U.S. Pat. Nos. 4,574,369 and 4,562,562 (FIG. 3). U.S. Pat. No. 4,143,402 shows a piezo-electric magnetic head control and support mechanism using two parallel piezo-electric-bender arms which are cantilevered from a support. The support contains the electrical connections to the piezo-electric arms. The magnetic head is moved in along one degree of freedom, i.e., transfers to the track being scanned.
Several configurations have been proposed and used for supporting either magnetic or optical transducers at the distal ends of cantilevered bendable piezo-electric support arms. An example of a four-arm support is shown in U.S. Pat. No. 4,475,179. An objective lens assembly is mounted between the distal ends of the four-cantilevered piezo-electric arms which respectively form a corner of a rectangle defined by the objective lens mount. A pair of the cantilevered piezo-electric arms disposed on one side of the objective lens optical axis cooperate to move the lens mount toward and away from a record medium. The pair of cantilevered piezo-electric arms on the side opposite of the optical axis to the first pair of such arms are disposed to actuate the lens holder orthogonally to the axis of the objective lens, i.e., for tracking and short seeks. Therefore, two of the arms provide for focus while two of the other arms provide for tracking and short seeks. This arrangement requires that each of the cantilevered piezo-electric arms be flexible along two axes along the lengths of the arms. The normal axis of the bending due to the internal piezo-electric forces and in the direction orthogonal to such normal bending action is found in known piezo-electric bending arms. A simpler arrangement is desired.
U.S. Pat. No. 4,616,355 teaches that piezo-electric bender supports, such as shown in FIGS. 4 through 8 of that patent, have difficulty with mechanical resonances induced into the assembly because of the piezo-electric bender arms. Examination of FIGS. 4-8 show a complex arrangement wherein the lens holder is supported through a pair of upstanding studs. Two sets of piezo-electric parallelogram-arranged arms are disposed between the studs and the lens holder such that the distal ends of the piezo-electric parallel arm assemblies support the lens respectively at one corner and at a center of one side of the holder. This arrangement appears to result in focus changes whenever tracking changes are made and tracking changes whenever focus changes are made. The patent further teaches that a magnetic actuator arrangement using two sets of parallelogram-oriented supports be employed. A pair of opposing upstanding frame supports support the outward ends of the dual parallelogram arrangements. The inner portion of the parallelogram arrangements support respective oppositely-facing sides of a lens holder. Coils for actuating the lens for tracking and focussing found in the lens holder itself provides magnetic forces which cause motion along the axis of the lens and transverse to the axis of the lens by flexing the support assembly at predetermined junctures between rigid arms. Again, it is desired to avoid such a complex and expensive holder arrangement.
U.S. Pat. No. 4,385,373 provides for a plurality of independently addressable piezo-electric bender elements for supporting a lens in an optical system. The bender elements are distorted along their lengths for pivoting the axis of the lens for performing tracking functions. The patent teaches that it is preferred to have four piezo-electric bender elements such as shown in FIGS. 7 and 8 which are coupled to a lens holder at the respective corners thereof. A similar arrangement is desired.
U.S. Pat. No. 4,441,128 shows a magnetic head support having two parallel bimorph bender elements supporting same. The head element is supported at the distal end of the bimorphs in such a way that the movement of the head along one degree of freedom is always along a straight line. Bimorphs are always maintained parallel to keep the head from turning or twisting. Two axes of motion are desired.
U.S. Pat. No. 4,258,398 shows another magnetic head support using two piezo-electric cantilevered bender beams. The beams are designed to flex in the direction of the track being scanned. They are spaced apart transversely to the track direction. The beams can be bent in opposite directions for adapting the orientation of the supported head for skew with respect to a plurality of tracks on a tape being scanned. Sum and difference error derived control signals actuate the cantilevered piezo-electric bender beams accommodating the skew. No tracking or track seeking fnnctions are performed.